Genetic versus chemoprotective activation of Nrf2 signaling: overlapping yet distinct gene expression profiles between Keap1 knockout and triterpenoid-treated mice

Inducers of Senescence, Toxic Compounds, and Senolytics: The Multiple Faces of Nrf2-Activating Phytochemicals in Cancer Adjuvant Therapy

The reactivation of senescence in cancer and the subsequent clearance of senescent cells are suggested as therapeutic intervention in the eradication of cancer. Several natural compounds that activate Nrf2 (nuclear factor erythroid-derived 2-related factor 2) pathway, which is involved in complex cytoprotective responses, have been paradoxically shown to induce cell death or senescence in cancer. Promoting the cytoprotective Nrf2 pathway may be desirable for chemoprevention, but it might be detrimental in later stages and advanced cancers. However, senolytic activity shown by some Nrf2-activating compounds could be used to target senescent cancer cells (particularly in aged immune-depressed organisms) that escape immunosurveillance. We herein describe in vitro and in vivo effects of fifteen Nrf2-interacting natural compounds (tocotrienols, curcumin, epigallocatechin gallate, quercetin, genistein, resveratrol, silybin, phenethyl isothiocyanate, sulforaphane, triptolide, allicin, berberine, piperlongumine, fisetin, and phloretin) on cellular senescence and discuss their use in adjuvant cancer therapy. In light of available literature, it can be concluded that the meaning and the potential of adjuvant therapy with natural compounds in humans remain unclear, also taking into account the existence of few clinical trials mostly characterized by uncertain results. Further studies are needed to investigate the therapeutic potential of those compounds that display senolytic activity.

Regulation of redox balance in cancer and T cells

Reactive oxygen species (ROS) mediate redox signaling necessary for numerous cellular functions. Yet, high levels of ROS in cells and tissues can cause damage and cell death. Therefore, regulation of redox homeostasis is essential for ROS-dependent signaling that does not incur cellular damage. Cells achieve this optimal balance by coordinating ROS production and elimination. In this Minireview, we discuss the mechanisms by which proliferating cancer and T cells maintain a carefully controlled redox balance. Greater insight into such redox biology may enable precisely targeted manipulation of ROS for effective medical therapies against cancer or immunological disorders.

Discovery of benzo[g]indoles as a novel class of non-covalent Keap1-Nrf2 protein-protein interaction inhibitor

The Keap1-Nrf2 system is an attractive target for drug discovery regarding various unmet medical needs. Only covalent inhibitors for protein-protein interaction (PPI) between Keap1 and Nrf2 to activate Nrf2 have been approved or are under clinical trials, but such electrophilic compounds lack selectivity. Therefore, specific non-covalent Keap1-Nrf2 PPI inhibitors are expected to be safer Nrf2 activators. We found a novel class of non-covalent Keap1-Nrf2 PPI inhibitor that has a benzo[g]indole skeleton and an indole-3-hydroxamic acid moiety and that exhibits significant PPI inhibitory activity. Additionally, the benzo[g]indole-3-carbohydrazide derivatives were newly prepared. The benzo[g]indole derivatives showed a stronger Keap1-Nrf2 PPI inhibitory activity than Cpd16, a previously reported non-covalent PPI inhibitor. Moreover, most of the PPI inhibitors showed a high metabolic stability in a human microsome system with a low cytotoxicity against HepG2 cell lines, which suggests that novel benzo[g]indole-type Keap1-Nrf2 PPI inhibitors are expected to be biological tools or lead compounds for Nrf2 activators.

A unique tolerizing dendritic cell phenotype induced by the synthetic triterpenoid CDDO-DFPA (RTA-408) is protective against EAE

Tolerogenic dendritic cells (DCs) have emerged as relevant clinical targets for the treatment of multiple sclerosis and other autoimmune disorders. However, the pathways essential for conferring the tolerizing DC phenotype and optimal methods for their induction remain an intense area of research. Triterpenoids are a class of small molecules with potent immunomodulatory activity linked to activation of Nrf2 target genes, and can also suppress the manifestations of experimental autoimmune encephalomyelitis (EAE). Here we demonstrate that DCs are a principal target of the immune modulating activity of triterpenoids in the context of EAE. Exposure of DCs to the new class of triterpenoid CDDO-DFPA (RTA-408) results in the induction of HO-1, TGF-β, and IL-10, as well as the repression of NF-κB, EDN-1 and pro-inflammatory cytokines IL-6, IL-12, and TNFα. CDDO-DFPA exposed DCs retained expression of surface ligands and capacity for antigen uptake but were impaired to induce Th1 and Th17 cells. TGF-β was identified as the factor mediating suppression of T cell proliferation by CDDO-DFPA pretreated DCs, which failed to passively induce EAE. These findings demonstrate the potential therapeutic utility of CDDO-DFPA in the treatment and prevention of autoimmune disorders, and its capacity to induce tolerance via modulation of the DC phenotype.

Biochemical characterization and anti-inflammatory properties of an isothiocyanate-enriched moringa (Moringa oleifera) seed extract

Moringa oleifera Lam. is a tropical plant, used for centuries as food and traditional medicine. The aim of this study was to develop, validate and biochemically characterize an isothiocyanate-enriched moringa seed extract (MSE), and to compare the anti-inflammatory effects of MSE-containing moringa isothiocyanate-1 (MIC-1) with a curcuminoid-enriched turmeric extract (CTE), and a material further enriched in its primary phytochemical, curcumin (curcumin-enriched material; CEM). MSE was prepared by incubating ground moringa seeds with water to allow myrosinase-catalyzed enzymatic formation of bioactive MIC-1, the predominant isothiocyanate in moringa seeds. Optimization of the extraction process yielded an extract of 38.9% MIC-1. Phytochemical analysis of MSE revealed the presence of acetylated isothiocyanates, phenolic glycosides unique to moringa, flavonoids, fats and fatty acids, proteins and carbohydrates. MSE showed a reduction in the carrageenan-induced rat paw edema (33% at 500 mg/kg MIC-1) comparable to aspirin (27% at 300 mg/kg), whereas CTE did not have any significant effect. In vitro, MIC-1 at 1 μM significantly reduced the production of nitric oxide (NO) and at 5 μM, the gene expression of LPS-inducible nitric oxide synthase (iNOS) and interleukins 1β and 6 (IL-1β and IL-6), whereas CEM did not show any significant activity at all concentrations tested. MIC-1 (10μM) was also more effective at upregulating the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) target genes NAD(P)H:quinone oxidoreductase 1 (NQO1), glutathione S-transferase pi 1 (GSTP1), and heme oxygenase 1 (HO1) than the CEM. Thus, in contrast to CTE and CEM, MSE and its major isothiocyanate MIC-1 displayed strong anti-inflammatory and antioxidant properties in vivo and in vitro, making them promising botanical leads for the mitigation of inflammatory-mediated chronic disorders.

Protection of Nrf2 against arsenite-induced oxidative damage is regulated by the cyclic guanosine monophosphate-protein kinase G signaling pathway

Arsenite has been shown to induce a variety of oxidative damage in mammalian cells. However, the mechanisms underlying cellular responses to its adverse effects remain unknown. We previously showed that the level of Nrf2, a nuclear transcription factor significantly increased in arsenite-treated human bronchial epithelial (HBE) cells suggesting that Nrf2 is involved in responding to arsenite-induced oxidative damage. To explore how Nrf2 can impact arsenite-induced oxidative damage, in this study, we examined Nrf2 activation and its regulation upon cellular arsenite exposure as well as its effects on arsenite-induced oxidative damage in HBE cells. We found that Nrf2 mRNA and protein levels were significantly increased by arsenite in a dose- and time-dependent manner. Furthermore, we showed that over-expression of Nrf2 significantly reduced the level of arsenite-induced oxidative damage in HBE cells including DNA damage, chromosomal breakage, lipid peroxidation and depletion of antioxidants. This indicates a protective role of Nrf2 against arsenite toxicity. This was further supported by the fact that activation of Nrf2 by its agonists, tertiary butylhydroquinone (t-BHQ) and sulforaphane (SFN) resulted in the same protective effects against arsenite toxicity. Moreover, we demonstrated that arsenite-induced activation of Nrf2 was mediated by the cyclic guanosine monophosphate (cGMP)-protein kinase G (PKG) signaling pathway. This is the first evidence showing that Nrf2 protects against arsenite-induced oxidative damage through the cGMP-PKG pathway. Our study suggests that activation of Nrf2 through the cGMP-PKG signaling pathway in HBE cells may be developed as a new strategy for prevention of arsenite toxicity. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 2004-2020, 2017.

Nrf2 targeting by sulforaphane: A potential therapy for cancer treatment

In the past decades, extensive studies have reported the potential chemopreventive activity of sulforaphane, an isothiocyanate derived from glucoraphanin, occurring in large amounts in Brassica genus plants. Sulforaphane was found to be active against several forms of cancer. A growing body of data shows that sulforaphane acts against cancer at different levels, from development to progression, through pleiotropic effects. In this review, we discuss the available experimental and clinical data on the potential therapeutic role of sulforaphane against cancer. Its effects range from the protection of cells from DNA damage to the modulation of the cell cycle via pro-apoptotic, anti-angiogenesis and anti-metastasis activities. At molecular level, sulforaphane modulates cellular homeostasis via the activation of the transcription factor Nrf2. Although data from clinical studies are limited, sulforaphane remains a good candidate in the adjuvant therapy based on natural molecules against several types of cancer.

Redox-based therapeutics in neurodegenerative disease

This review describes recent developments in the search for effective therapeutic agents that target redox homeostasis in neurodegenerative disease. The disruption to thiol redox homeostasis in Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis is discussed, together with the experimental strategies that are aimed at preventing, or at least minimizing, oxidative damage in these diseases. Particular attention is given to the potential of increasing antioxidant capacity by targeting the Nrf2 pathway, the development of inhibitors of NADPH oxidases that are likely candidates for clinical use, together with strategies to reduce nitrosative stress and mitochondrial dysfunction. We describe the shortcomings of compounds that hinder their progression to the clinic and evaluate likely avenues for future research.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

Low-Dose Dose-Response for In Vitro Nrf2-ARE Activation in Human HepG2 Cells

Kelch-like ECH-associated protein 1 (Keap1), nuclear factor erythroid 2-like factor 2-related factor 2 (Nrf2), and the antioxidant response element (ARE) are interacting components of a master regulatory signaling pathway that coordinates redox homeostasis, cytoprotective responses, and shifts in stem cell state. This study reexamined detailed dose–response (DR) data reported for in vitro Nrf2-ARE activation in human hepatoblastoma HepG2 cell lines containing either a ARE-bla or ARE-luc reporter at 12 different concentrations of each of 15 chemicals. The normalized study data were combined among chemicals exhibiting a positive response, yielding n = 531 (179) DR data for 9 (7) chemicals using the ARE-bla (ARE-luc) assay. Three-parameter linear/kth-power regression fits obtained to each combined set of ARE-bla- or ARE-luc-assay response data provided good fits (R² = .99 or .91, respectively, P_fit > .99) that each incorporate a highly significant negative initial linear slope (P = 4 × 10⁻⁵ or .00025) and an overall J-shaped DR pattern. Results from this reanalysis of high-resolution ARE response data support the hypothesis that nonlinear ARE-mediated adaptive cellular responses to oxidative stress are governed by an ultrasensitive molecular switch.

Glucoraphanin Ameliorates Obesity and Insulin Resistance Through Adipose Tissue Browning and Reduction of Metabolic Endotoxemia in Mice

Low-grade sustained inflammation links obesity to insulin resistance and nonalcoholic fatty liver disease (NAFLD). However, therapeutic approaches to improve systemic energy balance and chronic inflammation in obesity are limited. Pharmacological activation of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) alleviates obesity and insulin resistance in mice; however, Nrf2 inducers are not clinically available owing to safety concerns. Thus, we examined whether dietary glucoraphanin, a stable precursor of the Nrf2 inducer sulforaphane, ameliorates systemic energy balance, chronic inflammation, insulin resistance, and NAFLD in high-fat diet (HFD)-fed mice. Glucoraphanin supplementation attenuated weight gain, decreased hepatic steatosis, and improved glucose tolerance and insulin sensitivity in HFD-fed wild-type mice but not in HFD-fed Nrf2 knockout mice. Compared with vehicle-treated controls, glucoraphanin-treated HFD-fed mice had lower plasma lipopolysaccharide levels and decreased relative abundance of the gram-negative bacteria family Desulfovibrionaceae in their gut microbiomes. In HFD-fed mice, glucoraphanin increased energy expenditure and the protein expression of uncoupling protein 1 (Ucp1) in inguinal and epididymal adipose depots. Additionally, in this group, glucoraphanin attenuated hepatic lipogenic gene expression, lipid peroxidation, classically activated M1-like macrophage accumulation, and inflammatory signaling pathways. By promoting fat browning, limiting metabolic endotoxemia-related chronic inflammation, and modulating redox stress, glucoraphanin may mitigate obesity, insulin resistance, and NAFLD.

Oxidative Stress as a Critical Factor in Nonalcoholic Fatty Liver Disease Pathogenesis

SIGNIFICANCE

Nonalcoholic fatty liver disease (NAFLD), characterized by liver triacylglycerol build-up, has been growing in the global world in concert with the raised prevalence of cardiometabolic disorders, including obesity, diabetes, and hyperlipemia. Redox imbalance has been suggested to be highly relevant to NAFLD pathogenesis. Recent Advances: As a major health problem, NAFLD progresses to the more severe nonalcoholic steatohepatitis (NASH) condition and predisposes susceptible individuals to liver and cardiovascular disease. Although NAFLD represents the predominant cause of chronic liver disorders, the mechanisms of its development and progression remain incompletely understood, even if various scientific groups ascribed them to the occurrence of insulin resistance, dyslipidemia, inflammation, and apoptosis. Nevertheless, oxidative stress (OxS) more and more appears as the most important pathological event during NAFLD development and the hallmark between simple steatosis and NASH manifestation.

CRITICAL ISSUES

The purpose of this article is to summarize recent developments in the understanding of NAFLD, essentially focusing on OxS as a major pathogenetic mechanism. Various attempts to translate reactive oxygen species (ROS) scavenging by antioxidants into experimental and clinical studies have yielded mostly encouraging results.

FUTURE DIRECTIONS

Although augmented concentrations of ROS and faulty antioxidant defense have been associated to NAFLD and related complications, mechanisms of action and proofs of principle should be highlighted to support the causative role of OxS and to translate its concept into the clinic. Antioxid. Redox Signal. 26, 519-541.

Role of Nuclear Factor Erythroid 2-Related Factor 2 in Diabetic Nephropathy

Diabetic nephropathy (DN) is manifested as increased urinary protein level, decreased glomerular filtration rate, and final renal dysfunction. DN is the leading cause of end-stage renal disease worldwide and causes a huge societal healthcare burden. Since satisfied treatments are still limited, exploring new strategies for the treatment of this disease is urgently needed. Oxidative stress takes part in the initiation and development of DN. In addition, nuclear factor erythroid 2-related factor 2 (Nrf2) plays a key role in the cellular response to oxidative stress. Thus, activation of Nrf2 seems to be a new choice for the treatment of DN. In current review, we discussed and summarized the therapeutic effects of Nrf2 activation on DN from both basic and clinical studies.

Biochemical characterization and anti-inflammatory properties of an isothiocyanate-enriched moringa (Moringa oleifera) seed extract

Moringa oleifera Lam. is a tropical plant, used for centuries as food and traditional medicine. The aim of this study was to develop, validate and biochemically characterize an isothiocyanate-enriched moringa seed extract (MSE), and to compare the anti-inflammatory effects of MSE-containing moringa isothiocyanate-1 (MIC-1) with a curcuminoid-enriched turmeric extract (CTE), and a material further enriched in its primary phytochemical, curcumin (curcumin-enriched material; CEM). MSE was prepared by incubating ground moringa seeds with water to allow myrosinase-catalyzed enzymatic formation of bioactive MIC-1, the predominant isothiocyanate in moringa seeds. Optimization of the extraction process yielded an extract of 38.9% MIC-1. Phytochemical analysis of MSE revealed the presence of acetylated isothiocyanates, phenolic glycosides unique to moringa, flavonoids, fats and fatty acids, proteins and carbohydrates. MSE showed a reduction in the carrageenan-induced rat paw edema (33% at 500 mg/kg MIC-1) comparable to aspirin (27% at 300 mg/kg), whereas CTE did not have any significant effect. In vitro, MIC-1 at 1 μM significantly reduced the production of nitric oxide (NO) and at 5 μM, the gene expression of LPS-inducible nitric oxide synthase (iNOS) and interleukins 1β and 6 (IL-1β and IL-6), whereas CEM did not show any significant activity at all concentrations tested. MIC-1 (10μM) was also more effective at upregulating the nuclear factor (erythroid-derived 2)-like 2 (Nrf2) target genes NAD(P)H:quinone oxidoreductase 1 (NQO1), glutathione S-transferase pi 1 (GSTP1), and heme oxygenase 1 (HO1) than the CEM. Thus, in contrast to CTE and CEM, MSE and its major isothiocyanate MIC-1 displayed strong anti-inflammatory and antioxidant properties in vivo and in vitro, making them promising botanical leads for the mitigation of inflammatory-mediated chronic disorders.

Conservation of the Nrf2-Mediated Gene Regulation of Proteasome Subunits and Glucose Metabolism in Zebrafish

The Keap1-Nrf2 system is an evolutionarily conserved defense mechanism against oxidative and xenobiotic stress. Besides the exogenous stress response, Nrf2 has been found to regulate numerous cellular functions, including protein turnover and glucose metabolism; however, the evolutionary origins of these functions remain unknown. In the present study, we searched for novel target genes associated with the zebrafish Nrf2 to answer this question. A microarray analysis of zebrafish embryos that overexpressed Nrf2 revealed that 115 candidate genes were targets of Nrf2, including genes encoding proteasome subunits and enzymes involved in glucose metabolism. A real-time quantitative PCR suggested that the expression of 3 proteasome subunits (psma3, psma5, and psmb7) and 2 enzymes involved in glucose metabolism (pgd and fbp1a) were regulated by zebrafish Nrf2. We thus next examined the upregulation of these genes by an Nrf2 activator, diethyl maleate, using Nrf2 mutant zebrafish larvae. The results of real-time quantitative PCR and whole-mount in situ hybridization showed that all of these 5 genes were upregulated by diethyl maleate treatment in an Nrf2-dependent manner, especially in the liver. These findings implied that the Nrf2-mediated regulation of the proteasome subunits and glucose metabolism is evolutionarily conserved among vertebrates.

Withaferin A induces Nrf2-dependent protection against liver injury: Role of Keap1-independent mechanisms

Small molecules of plant origin offer presumptively safe opportunities to prevent carcinogenesis, mutagenesis and other forms of toxicity in humans. However, the mechanisms of action of such plant-based agents remain largely unknown. In recent years the stress responsive transcription factor Nrf2 has been validated as a target for disease chemoprevention. Withania somnifera (WS) is a herb used in Ayurveda (an ancient form of medicine in South Asia). In the recent past, withanolides isolated from WS, such as Withaferin A (WA) have been demonstrated to be preventive and therapeutic against multiple diseases in experimental models. The goals of this study are to evaluate withanolides such as WA as well as Withania somnifera root extract as inducers of Nrf2 signaling, to probe the underlying signaling mechanism of WA and to determine whether prevention of acetaminophen (APAP)-induced hepatic toxicity in mice by WA occurs in an Nrf2-dependent manner. We observed that WA profoundly protects wild-type mice but not Nrf2-disrupted mice against APAP hepatotoxicity. WA is a potent inducer of Nrf2-dependent cytoprotective enzyme expression both in vivo and in vitro. Unexpectedly, WA induces Nrf2 signaling at least in part, in a Keap1-independent, Pten/Pi3k/Akt-dependent manner in comparison to prototypical Nrf2 inducers, sulforaphane and CDDO-Im. The identification of WA as an Nrf2 inducer that can signal through a non-canonical, Keap1-independent pathway provides an opportunity to evaluate the role of other regulatory partners of Nrf2 in the dietary and pharmacological induction of Nrf2-mediated cytoprotection.

Discovery and Development of Kelch-like ECH-Associated Protein 1. Nuclear Factor Erythroid 2-Related Factor 2 (KEAP1:NRF2) Protein-Protein Interaction Inhibitors: Achievements, Challenges, and Future Directions

The transcription factor Nrf2 is the primary regulator of the cellular defense system, and enhancing Nrf2 activity has potential usages in various diseases, especially chronic age-related and inflammatory diseases. Recently, directly targeting Keap1-Nrf2 protein-protein interaction (PPI) has been an emerging strategy to selectively and effectively activate Nrf2. This Perspective summarizes the progress in the discovery and development of Keap1-Nrf2 PPI inhibitors, including the Keap1-Nrf2 regulatory mechanisms, biochemical techniques for inhibitor identification, and approaches for identifying peptide and small-molecule inhibitors, as well as discusses privileged structures and future directions for further development of Keap1-Nrf2 PPI inhibitors.

Small Maf deficiency recapitulates the liver phenotypes of Nrf1- and Nrf2-deficient mice

Nrf1 and Nrf2 (NF-E2-related factors 1 and 2, respectively) are transcription factors that belong to the Cap'n'collar (CNC) family and play critical roles in various tissues, including the liver. Liver-specific Nrf1 knockout mice show hepatic steatosis, accompanied by dysregulation of various metabolic genes. Nrf2 knockout mice show impairment in the induction of antioxidant and xenobiotic-metabolizing enzyme genes. Although it has been shown that small Maf (sMaf) proteins act as obligatory partners of CNC proteins, their precise contributions to the function of CNC proteins remain unclear especially in the context of adult liver functions. To address this issue, we generated mice that conditionally lack expression of all sMaf proteins in the liver. The liver-specific sMaf-deficient mice develop hepatic steatosis and dysregulation of genes involved in lipid and amino acid metabolism and proteasomal subunit expression. Importantly, the gene expression profiles in the sMaf-deficient livers share a strong similarity with those in Nrf1-deficient livers. In addition, the basal expression levels of a number of Nrf2 target genes were diminished in the sMaf-deficient livers. These results provide the first genetic evidence that sMaf proteins are indispensable for liver functions as heterodimeric partners for Nrf1 and Nrf2.

p62/SQSTM1-Dr. Jekyll and Mr. Hyde that prevents oxidative stress but promotes liver cancer

p62/SQSTM1 is a multifunctional signaling hub and autophagy adaptor with many binding partners, which allow it to activate mTORC1-dependent nutrient sensing, NF-κB-mediated inflammatory responses, and the NRF2-activated antioxidant defense. p62 recognizes polyubiquitin chains via its C-terminal domain and binds to LC3 via its LIR motif, thereby promoting the autophagic degradation of ubiquitinated cargos. p62 accumulates in many human liver diseases, including nonalcoholic steatohepatitis (NASH) and hepatocellular carcinoma (HCC), where it is a component of Mallory-Denk bodies and intracellular hyaline bodies. Chronic p62 elevation contributes to HCC development by preventing oncogene-induced senescence and death of cancer-initiating cells and enhancing their proliferation. In this review, we discuss p62-mediated signaling pathways and their roles in liver pathophysiology, especially NASH and HCC.

Novel Grb14-Mediated Cross Talk between Insulin and p62/Nrf2 Pathways Regulates Liver Lipogenesis and Selective Insulin Resistance

A long-standing paradox in the pathophysiology of metabolic diseases is the selective insulin resistance of the liver. It is characterized by a blunted action of insulin to reduce glucose production, contributing to hyperglycemia, while de novo lipogenesis remains insulin sensitive, participating in turn to hepatic steatosis onset. The underlying molecular bases of this conundrum are not yet fully understood. Here, we established a model of selective insulin resistance in mice by silencing an inhibitor of insulin receptor catalytic activity, the growth factor receptor binding protein 14 (Grb14) in liver. Indeed, Grb14 knockdown enhanced hepatic insulin signaling but also dramatically inhibited de novo fatty acid synthesis. In the liver of obese and insulin-resistant mice, downregulation of Grb14 markedly decreased blood glucose and improved liver steatosis. Mechanistic analyses showed that upon Grb14 knockdown, the release of p62/sqstm1, a partner of Grb14, activated the transcription factor nuclear factor erythroid-2-related factor 2 (Nrf2), which in turn repressed the lipogenic nuclear liver X receptor (LXR). Our study reveals that Grb14 acts as a new signaling node that regulates lipogenesis and modulates insulin sensitivity in the liver by acting at a crossroad between the insulin receptor and the p62-Nrf2-LXR signaling pathways.

The Dual Roles of NRF2 in Cancer

NRF2 has been traditionally considered as a tumor suppressor because its cytoprotective functions are deemed to be the main cellular defense mechanism against exogenous and endogenous insults, including xenobiotics and oxidative stress. However, several recent studies demonstrate that hyperactivation of the NRF2 pathway creates an environment that favors the survival of normal as well as malignant cells, protecting them against oxidative stress, chemotherapeutic agents, and radiotherapy. In a rapidly advancing field, this review summarizes some of the known mechanisms by which NRF2 can exert its oncogenic functions, and describes the current status of NRF2 inhibitors, providing a clear rationale for the consideration of NRF2 as a powerful putative therapeutic target in cancer treatment.

The Keap1-Nrf2-ARE Pathway As a Potential Preventive and Therapeutic Target: An Update

The Keap1-Nrf2-ARE ((Kelch-like ECH-Associating protein 1) nuclear factor erythroid 2 related factor 2-antioxidant response element) pathway is one of the most important defense mechanisms against oxidative and/or electrophilic stresses, and it is closely associated with inflammatory diseases, including cancer, neurodegenerative diseases, cardiovascular diseases, and aging. In recent years, progress has been made in strategies aimed at modulating the Keap1-Nrf2-ARE pathway. The Nrf2 activator DMF (Dimethylfumarates) has been approved by the FDA as a new first-line oral drug to treat patients with relapsing forms of multiple sclerosis, while a phase 3 study of another promising candidate, CDDO-Me, was terminated for safety reasons. Directly inhibiting Keap1-Nrf2 protein-protein interactions as a novel Nrf2-modulating strategy has many advantages over using electrophilic Nrf2 activators. The development of Keap1-Nrf2 protein-protein interaction inhibitors has become a topic of intense research, and potent inhibitors of this target have been identified. In addition, inhibiting Nrf2 activity has attracted an increasing amount of attention because it may provide an alternative cancer therapy. This review summarizes the molecular mechanisms and biological functions of the Keap1-Nrf2-ARE system. The main focus of this review is on recent progress in studies of agents that target the Keap1-Nrf2-ARE pathway and the therapeutic applications of such agents.

Nrf2 activation in the treatment of neurodegenerative diseases: a focus on its role in mitochondrial bioenergetics and function

The nuclear factor erythroid-derived 2 (NF-E2)-related factor 2 (Nrf2) is a transcription factor well-known for its function in controlling the basal and inducible expression of a variety of antioxidant and detoxifying enzymes. As part of its cytoprotective activity, increasing evidence supports its role in metabolism and mitochondrial bioenergetics and function. Neurodegenerative diseases are excellent candidates for Nrf2-targeted treatments. Most neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, frontotemporal dementia and Friedreich's ataxia are characterized by oxidative stress, misfolded protein aggregates, and chronic inflammation, the common targets of Nrf2 therapeutic strategies. Together with them, mitochondrial dysfunction is implicated in the pathogenesis of most neurodegenerative disorders. The recently recognized ability of Nrf2 to regulate intermediary metabolism and mitochondrial function makes Nrf2 activation an attractive and comprehensive strategy for the treatment of neurodegenerative disorders. This review aims to focus on the potential therapeutic role of Nrf2 activation in neurodegeneration, with special emphasis on mitochondrial bioenergetics and function, metabolism and the role of transporters, all of which collectively contribute to the cytoprotective activity of this transcription factor.

Emerging roles of Nrf2 signal in non-small cell lung cancer

Non-small cell lung cancer (NSCLC) causes considerable mortality in the world. Owing to molecular biological progress, treatments in adenocarcinoma have evolved revolutionarily while those in squamous lung cancer remain unsatisfied. Recent studies revealed high-frequency alteration of Kelch-like ECH-associated protein 1/nuclear factor erythroid 2-like factor 2 (Keap1/Nrf2) pathway within squamous lung cancer, attracting researchers to focus on this particular pathway. In NSCLC patients, deregulated Nrf2 signal is recognized as a common feature at both DNA and protein level. Emerging associations between Nrf2 and other pathways have been elucidated. MicroRNA was also implicated in the regulation of Nrf2. Agents activating or antagonizing Nrf2 showed an effect in preclinical researches, reflecting different effects of Nrf2 during tumor initiation and progression. Prognostic evaluation demonstrated a negative impact of Nrf2 signal on NSCLC patients’ survival. Considering the importance of Nrf2 signal in NSCLC, further studies are required in the future.

Green tea extract provides extensive Nrf2-independent protection against lipid accumulation and NFκB pro- inflammatory responses during nonalcoholic steatohepatitis in mice fed a high-fat diet

SCOPE

Green tea extract (GTE) reduces liver steatosis and inflammation during nonalcoholic steatohepatitis (NASH). We hypothesized GTE would mitigate NASH in a nuclear factor erythroid-2-related-factor-2 (Nrf2)-dependent manner in a high fat (HF) induced model.

METHODS AND RESULTS

Nrf2-null and wild-type (WT) mice were fed an HF diet containing 0 or 2% GTE for eight weeks prior to assessing parameters of NASH. Compared to WT mice, Nrf2-null mice had increased serum alanine aminotransferase, hepatic triglyceride, expression of free fatty acid uptake and lipogenic genes, malondialdehyde and NFκB phosphorylation and expression of pro-inflammatory genes. In WT mice, GTE increased Nrf2 and NADPH:quinone oxidoreductase-1 mRNA, and lowered hepatic steatosis, lipid uptake and lipogenic gene expression, malondialdehyde, and NFκB-dependent inflammation. In Nrf2-null mice, GTE lowered NFκB phosphorylation and TNF-α and MCP1 mRNA to levels observed in WT mice fed GTE whereas hepatic triglyceride and lipogenic genes were lowered only to those of WT mice fed no GTE. Malondialdehyde was lowered in Nrf2-null mice fed GTE, but not to levels of WT mice, and without improving the hepatic antioxidants α-tocopherol, ascorbic acid and uric acid.

CONCLUSION

Nrf2 deficiency exacerbates NASH whereas anti-inflammatory and hypolipidemic activities of GTE likely occur largely independent of Nrf2 signaling.

Keap1/Nrf2 pathway activation leads to a repressed hepatic gluconeogenic and lipogenic program in mice on a high-fat diet

The Keap1/Nrf2 pathway, known to regulate the expression of a series of cytoprotective and antioxidant genes, has been studied in the context of obesity and type 2 diabetes; diseases that are characterized by chronic oxidative stress. There is increasing evidence, however, that the transcription factor Nrf2 can crosstalk with pathways not directly related to cytoprotection. Our present work focuses on the effect of Nrf2 on hepatic gluconeogenesis and lipogenesis, two metabolic processes which are dysregulated in the obese/diabetic state. To this end, a genetic mouse model of Nrf2 pathway activation was used (Keap1-hypo; both Keap1 alleles are hypomorphic) and was exposed to a 3-month high-fat diet along with the relevant control wild-type mice. The Keap1-hypo mice were partially protected from obesity, had lower fasting glucose and insulin levels and developed less liver steatosis compared to the wild-type. Key gluconeogenic and lipogenic enzymes were repressed in the Keap1-hypo livers with concomitant activated Ampk signaling. Primary Keap1-hypo hepatocyte cultures also show increased Ampk signaling and repressed glucose production. In conclusion, increased Keap1/Nrf2 signaling in the liver is accompanied by repressed gluconeogenesis and lipogenesis that can, at least partially, explain the ameliorated diabetic phenotype in the Keap1-hypo mice.

Effects of Silicon vs. Hydroxytyrosol-Enriched Restructured Pork on Liver Oxidation Status of Aged Rats Fed High-Saturated/High-Cholesterol Diets

Background

Pork is an essential component of the diet that has been linked with major degenerative diseases and development of non-alcoholic steatohepatitis (NASH). Previous studies have. Previous studies have demonstrated the in vitro antioxidant activity of silicon (Si). Furthermore, when Si is added to restructured pork (RP) strongly counterbalances the negative effect of high-cholesterol-ingestion, acting as an active hypocholesterolemic and hypolipemic dietary ingredient in aged rats.

Objective

This study was designed to evaluate the effects of Si vs hydroxytyrosol (HxT) RP on liver antioxidant defense in aged rats fed cholesterol-enriched high saturated/high cholesterol diets as a NASH model.

Methods

Four diets were prepared: Control RP diet (C) with non-added cholesterol; Cholesterol-enriched high-saturated/high-cholesterol control RP diet (CHOL-C) with added cholesterol and cholic acid; Si- or HxT-RP cholesterol-enriched high-saturated/high-cholesterol diets (CHOL-Si and CHOL-HxT). Groups of six male Wistar rats (1-yr old) were fed these modified diets for eight weeks. Total cholesterol, hepatosomatic index, liver Nrf2 and antioxidant (CAT, SOD, GSH, GSSG, GR, GPx) markers were determined.

Results

Both CHOL-Si and CHOL-HxT diets enhanced the liver antioxidant status, reduced hepatosomatic index and increased SOD actvity. Hydrogen peroxide removal seemed to be involved, explaining that the value of redox index was even lower than C without changing the CAT activity. CHOL-Si results were quite better than CHOL-HxT in most measured parameters.

Conclusions

Our study suggests that Si incorporated into RP matrix was able to counterbalance, more efficiently than HxT, the deleterious effect of consuming a high-saturated/high-cholesterol diet, by improving the liver antioxidant defenses in the context of NASH.

CDDO and Its Role in Chronic Diseases

There has been a continued interest in translational research focused on both natural products and manipulation of functional groups on these compounds to create novel derivatives with higher desired activities. Oleanolic acid, a component of traditional Chinese medicine used in hepatitis therapy, was modified by chemical processes to form 2-cyano-3,12-dioxoolean-1,9-dien-28-oic acid (CDDO). This modification increased anti-inflammatory activity significantly and additional functional groups on the CDDO backbone have shown promise in treating conditions ranging from kidney disease to obesity to diabetes. CDDO's therapeutic effect is due to its upregulation of the master antioxidant transcription factor Nuclear factor erythroid 2-related factor 2 (Nrf2) through conformational change of Nrf2-repressing, Kelch-like erythroid cell-derived protein with CNC homology-associated protein 1 (Keap1) and multiple animal and human studies have verified subsequent activation of Nrf2-controlled antioxidant genes via upstream Antioxidant Response Element (ARE) regions. At the present time, positive results have been obtained in the laboratory and clinical trials with CDDO derivatives treating conditions such as lung injury, inflammation and chronic kidney disease. However, clinical trials for cancer and cardiovascular disease have not shown equally positive results and further exploration of CDDO and its derivatives is needed to put these shortcomings into context for the purpose of future therapeutic modalities.

The Nrf2/HO-1 Axis in Cancer Cell Growth and Chemoresistance

The transcription factor, nuclear factor erythroid 2 p45-related factor 2 (Nrf2), acts as a sensor of oxidative or electrophilic stresses and plays a pivotal role in redox homeostasis. Oxidative or electrophilic agents cause a conformational change in the Nrf2 inhibitory protein Keap1 inducing the nuclear translocation of the transcription factor which, through its binding to the antioxidant/electrophilic response element (ARE/EpRE), regulates the expression of antioxidant and detoxifying genes such as heme oxygenase 1 (HO-1). Nrf2 and HO-1 are frequently upregulated in different types of tumours and correlate with tumour progression, aggressiveness, resistance to therapy, and poor prognosis. This review focuses on the Nrf2/HO-1 stress response mechanism as a promising target for anticancer treatment which is able to overcome resistance to therapies.

Mechanisms of activation of the transcription factor Nrf2 by redox stressors, nutrient cues, and energy status and the pathways through which it attenuates degenerative disease

Nuclear factor-erythroid 2 p45-related factor 2 (Nrf2) regulates the basal and stress-inducible expression of a battery of genes encoding key components of the glutathione-based and thioredoxin-based antioxidant systems, as well as aldo-keto reductase, glutathione S-transferase, and

NAD(P)H

quinone oxidoreductase-1 drug-metabolizing isoenzymes along with multidrug-resistance-associated efflux pumps. It therefore plays a pivotal role in both intrinsic resistance and cellular adaptation to reactive oxygen species (ROS) and xenobiotics. Activation of Nrf2 can, however, serve as a double-edged sword because some of the genes it induces may contribute to chemical carcinogenesis by promoting futile redox cycling of polycyclic aromatic hydrocarbon metabolites or confer resistance to chemotherapeutic drugs by increasing the expression of efflux pumps, suggesting its cytoprotective effects will vary in a context-specific fashion. In addition to cytoprotection, Nrf2 also controls genes involved in intermediary metabolism, positively regulating those involved in NADPH generation, purine biosynthesis, and the β-oxidation of fatty acids, while suppressing those involved in lipogenesis and gluconeogenesis. Nrf2 is subject to regulation at multiple levels. Its ability to orchestrate adaptation to oxidants and electrophiles is due principally to stress-stimulated modification of thiols within one of its repressors, the Kelch-like ECH-associated protein 1 (Keap1), which is present in the cullin-3 RING ubiquitin ligase (CRL) complex CRLKeap1. Thus modification of Cys residues in Keap1 blocks CRLKeap1 activity, allowing newly translated Nrf2 to accumulate rapidly and induce its target genes. The ability of Keap1 to repress Nrf2 can be attenuated by p62/sequestosome-1 in a mechanistic target of rapamycin complex 1 (mTORC1)-dependent manner, thereby allowing refeeding after fasting to increase Nrf2-target gene expression. In parallel with repression by Keap1, Nrf2 is also repressed by β-transducin repeat-containing protein (β-TrCP), present in the Skp1-cullin-1-F-box protein (SCF) ubiquitin ligase complex SCFβ-TrCP. The ability of SCFβ-TrCP to suppress Nrf2 activity is itself enhanced by prior phosphorylation of the transcription factor by glycogen synthase kinase-3 (GSK-3) through formation of a DSGIS-containing phosphodegron. However, formation of the phosphodegron in Nrf2 by GSK-3 is inhibited by stimuli that activate protein kinase B (PKB)/Akt. In particular, PKB/Akt activity can be increased by phosphoinositide 3-kinase and mTORC2, thereby providing an explanation of why antioxidant-responsive element-driven genes are induced by growth factors and nutrients. Thus Nrf2 activity is tightly controlled via CRLKeap1 and SCFβ-TrCP by oxidative stress and energy-based signals, allowing it to mediate adaptive responses that restore redox homeostasis and modulate intermediary metabolism. Based on the fact that Nrf2 influences multiple biochemical pathways in both positive and negative ways, it is likely its dose-response curve, in terms of susceptibility to certain degenerative disease, is U-shaped. Specifically, too little Nrf2 activity will lead to loss of cytoprotection, diminished antioxidant capacity, and lowered β-oxidation of fatty acids, while conversely also exhibiting heightened sensitivity to ROS-based signaling that involves receptor tyrosine kinases and apoptosis signal-regulating kinase-1. By contrast, too much Nrf2 activity disturbs the homeostatic balance in favor of reduction, and so may have deleterious consequences including overproduction of reduced glutathione and NADPH, the blunting of ROS-based signal transduction, epithelial cell hyperplasia, and failure of certain cell types to differentiate correctly. We discuss the basis of a putative U-shaped Nrf2 dose-response curve in terms of potentially competing processes relevant to different stages of tumorigenesis.

The emerging role of Nrf2 in mitochondrial function

The transcription factor NF-E2 p45-related factor 2 (Nrf2; gene name NFE2L2) allows adaptation and survival under conditions of stress by regulating the gene expression of diverse networks of cytoprotective proteins, including antioxidant, anti-inflammatory, and detoxification enzymes as well as proteins that assist in the repair or removal of damaged macromolecules. Nrf2 has a crucial role in the maintenance of cellular redox homeostasis by regulating the biosynthesis, utilization, and regeneration of glutathione, thioredoxin, and NADPH and by controlling the production of reactive oxygen species by mitochondria and NADPH oxidase. Under homeostatic conditions, Nrf2 affects the mitochondrial membrane potential, fatty acid oxidation, availability of substrates (NADH and FADH2/succinate) for respiration, and ATP synthesis. Under conditions of stress or growth factor stimulation, activation of Nrf2 counteracts the increased reactive oxygen species production in mitochondria via transcriptional upregulation of uncoupling protein 3 and influences mitochondrial biogenesis by maintaining the levels of nuclear respiratory factor 1 and peroxisome proliferator-activated receptor γ coactivator 1α, as well as by promoting purine nucleotide biosynthesis. Pharmacological Nrf2 activators, such as the naturally occurring isothiocyanate sulforaphane, inhibit oxidant-mediated opening of the mitochondrial permeability transition pore and mitochondrial swelling. Curiously, a synthetic 1,4-diphenyl-1,2,3-triazole compound, originally designed as an Nrf2 activator, was found to promote mitophagy, thereby contributing to the overall mitochondrial homeostasis. Thus, Nrf2 is a prominent player in supporting the structural and functional integrity of the mitochondria, and this role is particularly crucial under conditions of stress.

Frugal chemoprevention: targeting Nrf2 with foods rich in sulforaphane

With the properties of efficacy, safety, tolerability, practicability and low cost, foods containing bioactive phytochemicals are gaining significant attention as elements of chemoprevention strategies against cancer. Sulforaphane [1-isothiocyanato-4-(methylsulfinyl)butane], a naturally occurring isothiocyanate produced by cruciferous vegetables such as broccoli, is found to be a highly promising chemoprevention agent against not only a variety of cancers such as breast, prostate, colon, skin, lung, stomach or bladder, but also cardiovascular disease, neurodegenerative diseases, and diabetes. For reasons of experimental exigency, preclinical studies have focused principally on sulforaphane itself, while clinical studies have relied on broccoli sprout preparations rich in either sulforaphane or its biogenic precursor, glucoraphanin. Substantive subsequent evaluation of sulforaphane pharmacokinetics and pharmacodynamics has been undertaken using either pure compound or food matrices. Sulforaphane affects multiple targets in cells. One key molecular mechanism of action for sulforaphane entails activation of the Nrf2-Keap1 signaling pathway although other actions contribute to the broad spectrum of efficacy in different animal models. This review summarizes the current status of pre-clinical chemoprevention studies with sulforaphane and highlights the progress and challenges for the application of foods rich in sulforaphane and/or glucoraphanin in the arena of clinical chemoprevention.

Cancer Cell Growth Is Differentially Affected by Constitutive Activation of NRF2 by KEAP1 Deletion and Pharmacological Activation of NRF2 by the Synthetic Triterpenoid, RTA 405

Synthetic triterpenoids are antioxidant inflammation modulators (AIMs) that exhibit broad anticancer activity. AIMs bind to KEAP1 and inhibit its ability to promote NRF2 degradation. As a result, NRF2 increases transcription of genes that restore redox balance and reduce inflammation. AIMs inhibit tumor growth and metastasis by increasing NRF2 activity in the tumor microenvironment and by modulating the activity of oncogenic signaling pathways, including NF-κB, in tumor cells. Accumulating evidence suggests that KEAP1 loss or mutation--which results in high levels of sustained NRF2 activity--may promote cancer growth and increase chemoresistance. Loss of KEAP1 also increases the levels of other oncogenic proteins, including IKKβ and BCL2. The apparent survival advantage provided to some tumor cells by loss of functional KEAP1 raises the question of whether pharmacological inhibition of KEAP1 could promote tumor growth. To address this issue, we characterized the basal levels of KEAP1 and NRF2 in a panel of human tumor cell lines and profiled the activity of an AIM, RTA 405. We found that in tumor cell lines with low or mutant KEAP1, and in Keap1-/- murine embryonic fibroblasts, multiple KEAP1 targets including NRF2, IKKβ, and BCL2 were elevated. Keap1-/- murine embryonic fibroblasts also had higher rates of proliferation and colony formation than their wild-type counterparts. In cells with functional KEAP1, RTA 405 increased NRF2 levels, but not IKKβ or BCL2 levels, and did not increase cell proliferation or survival. Moreover, RTA 405 inhibited growth at similar concentrations in cells with different basal NRF2 activity levels and in cells with wild-type or mutant KRAS. Finally, pre-treatment with RTA 405 did not protect tumor cells from doxorubicin- or cisplatin-mediated growth inhibition. Collectively, these data demonstrate that pharmacological activation of NRF2 by AIMs is distinct from genetic activation and does not provide a growth or survival advantage to tumor cells.

The complexity of the Nrf2 pathway: beyond the antioxidant response

The NF-E2-related factor 2 (Nrf2)-mediated signalling pathway provides living organisms an efficient and pivotal line of defensive to counteract environmental insults and endogenous stressors. Nrf2 coordinates the basal and inducible expression of antioxidant and Phase II detoxification enzymes to adapt to different stress conditions. The stability and cellular distribution of Nrf2 is tightly controlled by its inhibitory binding protein Kelch-like ECH-associated protein 1. Nrf2 signalling is also regulated by posttranslational, transcriptional, translational and epigenetic mechanisms, as well as by other protein partners, including p62, p21 and IQ motif-containing GTPase activating protein 1. Many studies have demonstrated that Nrf2 is a promising target for preventing carcinogenesis and other chronic diseases, including cardiovascular diseases, neurodegenerative diseases and pulmonary injury. However, constitutive activation of Nrf2 in advanced cancer cells may confer drug resistance. Here, we review the molecular mechanisms of Nrf2 signalling, the diverse classes of Nrf2 activators, including bioactive nutrients and other chemicals, and the cellular functions and disease relevance of Nrf2 and discuss the dual role of Nrf2 in different contexts.

Changing gears in Nrf1 research, from mechanisms of regulation to its role in disease and prevention

The "cap'n'collar" bZIP transcription factor Nrf1 heterodimerizes with small Maf proteins to bind to the Antioxidant Response Element/Electrophile Response Element to transactivate antioxidant enzyme, phase 2 detoxification enzyme and proteasome subunit gene expression. Nrf1 specifically regulates pathways in lipid metabolism, amino acid metabolism, proteasomal degradation, the citric acid cycle, and the mitochondrial respiratory chain. Nrf1 is maintained in the endoplasmic reticulum (ER) in an inactive glycosylated state. Activation involves retrotranslocation from the ER lumen to the cytoplasm, deglycosylation and partial proteolytic processing to generate the active forms of Nrf1. Recent evidence has revealed how this factor is regulated and its involvement in various metabolic diseases. This review outlines Nrf1 structure, function, regulation and its links to insulin resistance, diabetes and inflammation. The glycosylation/deglycosylation of Nrf1 is controlled by glucose levels. Nrf1 glycosylation affects its control of glucose transport, glycolysis, gluconeogenesis and lipid metabolism.

Interplay between cytosolic disulfide reductase systems and the Nrf2/Keap1 pathway

NADPH transfers reducing power from bioenergetic pathways to thioredoxin reductase-1 (TrxR1) and glutathione reductase (GR) to support essential reductive systems. Surprisingly, it was recently shown that mouse livers lacking both TrxR1 and GR ('TR/GR-null') can sustain redox (reduction-oxidation) homoeostasis using a previously unrecognized NADPH-independent source of reducing power fuelled by dietary methionine. The NADPH-dependent systems are robustly redundant in liver, such that disruption of either TrxR1 or GR alone does not cause oxidative stress. However, disruption of TrxR1 induces transcription factor Nrf2 (nuclear factor erythroid-derived 2-like-2) whereas disruption of GR does not. This suggests the Nrf2 pathway responds directly to the status of the thioredoxin-1 (Trx1) system. The proximal regulator of Nrf2 is Keap1 (Kelch-like ECH-associated protein-1), a cysteine (Cys)-rich protein that normally interacts transiently with Nrf2, targeting it for degradation. During oxidative stress, this interaction is stabilized, preventing degradation of newly synthesized Nrf2, thereby allowing Nrf2 accumulation. Within the Trx1 system, TrxR1 and peroxiredoxins (Prxs) contain some of the most reactive nucleophilic residues in the cell, making them likely targets for oxidants or electrophiles. We propose that Keap1 activity and therefore Nrf2 is regulated by interactions of Trx1 system enzymes with oxidants. In TR/GR-null livers, Nrf2 activity is further induced, revealing that TrxR-independent systems also repress Nrf2 and these might be induced by more extreme challenges.

Keap1/Nrf2 pathway in the frontiers of cancer and non-cancer cell metabolism

Cancer cells adapt their metabolism to their increased needs for energy and substrates for protein, lipid and nucleic acid synthesis. Nuclear erythroid factor 2-like 2 (Nrf2) pathway is usually activated in cancers and has been suggested to promote cancer cell survival mainly by inducing a large battery of cytoprotective genes. This mini review focuses on metabolic pathways, beyond cytoprotection, which can be directly or indirectly regulated by Nrf2 in cancer cells to affect their survival. The pentose phosphate pathway (PPP) is enhanced by Nrf2 in cancers and aids their growth. PPP has also been found to be up-regulated in non-cancer tissues and other pathways, such as de novo lipogenesis, have been found to be repressed after activation of the Nrf2 pathway. The importance of these Nrf2-regulated metabolic pathways in cancer compared with non-cancer state remains to be determined. Last but not least, the importance of context about Nrf2 and cancer is highlighted as the Nrf2 pathway may be activated in cancers but its pharmacological activators are useful in chemoprevention.

Novel synthetic pyridyl analogues of CDDO-Imidazolide are useful new tools in cancer prevention

Two new analogues of CDDO-Imidazolide (CDDO-Im), namely 1-[2-Cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]-4(-pyridin-2-yl)-1H-imidazole ("CDDO-2P-Im") and 1-[2-Cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]-4(-pyridin-3-yl)-1H-imidazole ("CDDO-3P-Im") have been synthesized and tested for their potential use as chemopreventive drugs. At nanomolar concentrations, they were equipotent to CDDO-Im for inducing differentiation and apoptosis in U937 leukemia cells. As inflammation and oxidative stress contribute to carcinogenesis, we also assessed their cytoprotective potential. The new compounds suppressed inducible nitric oxide synthase (iNOS) expression in RAW264.7 macrophage-like cells and significantly elevated heme oxygenase-1 (HO-1) and quinone reductase (NQO1) mRNA and protein levels in various mouse tissues in vivo. Most importantly, pharmacokinetic studies performed in vitro in human plasma and in vivo showed that each new analogue was more stable than CDDO-Im. Much higher concentrations of the new derivatives were found in mouse liver, lung, pancreas and kidney after gavage in contrast to CDDO-Im. Because of their better bioavailability and their excellent anti-inflammatory profile in vitro, CDDO-2P-Im and CDDO-3P-Im were tested for prevention in a highly relevant mouse lung cancer model, in which A/J mice develop lung carcinomas after injection of vinyl carbamate, a potent carcinogen. CDDO-2P-Im and CDDO-3P-Im were as effective as CDDO-Im for reducing the size and the severity of the lung tumors.

The Dual Role of Nrf2 in Nonalcoholic Fatty Liver Disease: Regulation of Antioxidant Defenses and Hepatic Lipid Metabolism

Nonalcoholic fatty liver disease (NAFLD) is a progressive liver disease with ever-growing incidence in the industrialized world. It starts with the simple accumulation of lipids in the hepatocyte and can progress to the more severe nonalcoholic steatohepatitis (NASH), which is associated with inflammation, fibrosis, and cirrhosis. There is increasing awareness that reactive oxygen species and electrophiles are implicated in the pathogenesis of NASH. Transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) is a positive regulator of the expression of a battery of genes involved in the protection against oxidative/electrophilic stress. In rodents, Nrf2 is also known to participate in hepatic fatty acid metabolism, as a negative regulator of genes that promote hepatosteatosis. We review relevant evidence in the literature that these two mechanisms may contribute to the protective role of Nrf2 in the development of hepatic steatosis and in the progression to steatohepatitis, particularly in young animals. We propose that age may be a key to explain contradictory findings in the literature. In summary, Nrf2 mediates the crosstalk between lipid metabolism and antioxidant defense mechanisms in experimental models of NAFLD, and the nutritional or pharmacological induction of Nrf2 represents a promising potential new strategy for its prevention and treatment.

RTA 408, A Novel Synthetic Triterpenoid with Broad Anticancer and Anti-Inflammatory Activity

Semi-synthetic triterpenoids are antioxidant inflammation modulator (AIM) compounds that inhibit tumor cell growth and metastasis. Compounds in the AIM class bind to Keap1 and attenuate Nrf2 degradation. In the nucleus, Nrf2 increases antioxidant gene expression and reduces pro-inflammatory gene expression. By increasing Nrf2 activity, AIMs reduce reactive oxygen species and inflammation in the tumor microenvironment, which reverses tumor-mediated immune evasion and inhibits tumor growth and metastasis. AIMs also directly inhibit tumor cell growth by modulating oncogenic signaling pathways, such as IKKβ/NF-κB. Here, we characterized the in vitro antioxidant, anti-inflammatory, and anticancer activities of RTA 408, a novel AIM that is currently being evaluated in patients with advanced malignancies. At low concentrations (≤ 25 nM), RTA 408 activated Nrf2 and suppressed nitric oxide and pro-inflammatory cytokine levels in interferon-γ-stimulated RAW 264.7 macrophage cells. At higher concentrations, RTA 408 inhibited tumor cell growth (GI₅₀ = 260 ± 74 nM) and increased caspase activity in tumor cell lines, but not in normal primary human cells. Consistent with the direct effect of AIMs on IKKβ, RTA 408 inhibited NF-κB signaling and decreased cyclin D1 levels at the same concentrations that inhibited cell growth and induced apoptosis. RTA 408 also increased CDKN1A (p21) levels and JNK phosphorylation. The in vitro activity profile of RTA 408 is similar to that of bardoxolone methyl, which was well-tolerated by patients at doses that demonstrated target engagement. Taken together, these data support clinical evaluation of RTA 408 for cancer treatment.

Mangiferin activates the Nrf2-ARE pathway and reduces etoposide-induced DNA damage in human umbilical cord mononuclear blood cells

CONTEXT

Mangiferin (2-C-β-d-gluco-pyranosyl-1,3,6,7-tetrahydroxyxanthone) is a well-known natural antioxidant distributed in various plants of the Anacardiaceae and Gentianaceae families. Mangiferin can inhibit carcinogen-induced lung or colon tumor formation in experimental animals. However, the molecular mechanisms of its chemopreventive activity remain unexplored.

OBJECTIVE

This study aimed to investigate the effects of mangiferin on chemical carcinogen-induced DNA damage and Nrf2-ARE signaling in hematopoietic cells.

MATERIALS AND METHODS

Mononuclear cells (MNCs) were isolated from human umbilical cord blood (hUCB). DNA damage was evaluated by comet and micronucleus assays. The expression of Nrf2 and NQO1 was examined by immunofluorescence and western blotting. An electrophoretic mobility shift assay (EMSA) was used to detect the binding activity of Nrf2 with NQO1-ARE sequences.

RESULTS

We found that mangiferin treatment significantly reduced DNA damage in etoposide-treated MNCs, which was verified by decreased olive tail moment (OTM) and micronucleus (MN) frequency. Mangiferin treatment significantly promoted Nrf2 translocation into the nucleus and increased nuclear Nrf2 expression. Moreover, NQO1, an Nrf2 signaling target, was significantly upregulated by mangiferin treatment, and the binding activity of Nrf2 with NQO1-ARE sequences was elevated after mangiferin treatment.

DISCUSSION AND CONCLUSION

Mangiferin activated Nrf2 signaling, upregulated NQO1 expression, and significantly reduced etoposide-induced DNA damage. Thus, mangiferin is a potential cytoprotective agent for hematopoietic cells.